I know ESPN occasionally shows various surface differences during matches (change-overs, etc). They just showed the hight difference of the average ball bounces on clay vs grass with fancy graphics and all.

One thing I noted was the very little difference with bounce height between a flat stroke and heavy top spin stroke on grass court. In fact, average bounce height is slightly less. Maybe this partly explains why Rafa's strokes seem to be less effective on grass. They are slow and just sit up at a comortable height. Would be interesting to see what the velocity is with heavy topspin strokes on grass court compared to clay too.